(a) Field of the Invention
The present invention relates to an automatic transmission for a vehicle, and more particularly, to a planetary gear train of an automatic transmission of a vehicle that improves power delivery performance and reduces fuel consumption by achieving nine forward speed stages using a minimum number of constituent elements.
(b) Description of the Related Art
Research regarding realizing more shift-stages of an automatic transmission is being conducted to achieve enhancement of fuel consumption and improved drivability, and recently, increase of oil price is triggering a competition in enhancing fuel consumption of a vehicle. In particular, research regarding an engine has been conducted to achieve weight reduction and to enhance fuel consumption by downsizing and research regarding an automatic transmission has been conducted to simultaneously provide improved drivability and fuel consumption by achieving more shift stages.
To achieve more shift stages for an automatic transmission, the number of parts is typically increased, which may deteriorate installability, production cost, weight and/or power flow efficiency. Therefore, to maximally enhance fuel consumption of an automatic transmission having more shift stages, it is important for improved efficiency to be derived by a smaller number of parts. In this respect, an eight-speed automatic transmission has been recently introduced, and a planetary gear train for an automatic transmission enabling more shift stages is being researched.
An automatic transmission of eight or more shift-stages typically includes three to four planetary gear sets and five to six control elements (frictional elements), and may easily become lengthy, thereby deteriorating installability. In this regard, disposing planetary gear sets in parallel or employing dog clutches instead of wet-type control elements has been contemplated. However, such an arrangement may not be widely applicable, and using dog clutches may deteriorate shift-feel.
The above information disclosed in this section is merely for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.